In the process of analyzing a sample by mass spectrometry (MS), the MS system first ionizes the sample to create analyte ions. The MS system then transfers the ions into a mass analyzer, and the mass analyzer resolves the ions on the basis of the ions' differing mass-to-charge (m/z) ratios. An ion detector measures the abundance of the ions at each m/z ratio detected. The MS system then processes signals outputted by the ion detector to generate mass (m/z) spectra that provide quantitative and qualitative information regarding the components of the sample (e.g., compounds, isomers, elements, etc.).
The mass analyzer operates in a controlled high-vacuum environment, for example at 10−6 to 10−9 Torr. In some MS systems, the ion source (where ionization of the sample is performed) also operates at a vacuum pressure. In other MS systems, such as when coupled to a liquid chromatography (LC) instrument (an LC-MS system), the ion source operates at or around atmospheric pressure. An MS system utilizing an atmospheric pressure ionization (API) source requires an interface between the API source and the evacuated regions of the MS system in which the mass analyzer and other devices are located. The interface needs to effectively isolate the atmospheric-pressure region where the ions are created from the evacuated regions where the ions are processed, while at the same time provide a way to efficiently transport the ions into the evacuated regions after they are created.
A capillary tube is often utilized to transfer the ions from the API source into the first vacuum region of the MS system. The capillary tube has a small inside bore, the inside diameter of which may range from a fraction of a millimeter (mm) to a few millimeters. The capillary tube extends through the boundary between the API source and the first vacuum region, whereby the tube's entrance is exposed to the ionization region of the API source and the tube's exit is exposed to the first vacuum region. Ions and gas in the API source are drawn into the tube's entrance, transported through the tube's bore, and emitted from the tube's exit into the first vacuum region. Ion optics guide the ions further into the MS system and ultimately to the mass analyzer. The capillary tube may be metal. Alternatively, the capillary tube may be glass with an electrically resistive property (coating or bulk resistance) to allow the tube's entrance to be placed at a relatively high voltage level while the tube's exit is maintained at a relatively low voltage level. In this case, the ions are effectively transported through the tube's bore because the gas drag forces on the ions in the capillary tube greatly exceed the ion mobility (electric) forces on the ions in the presence of the internal electric field in the capillary tube.
However, the capillary tube has a tendency to become contaminated after extended use, such as may be due to ion diffusion and space-charge repulsion, and thus periodically requires cleaning or even replacement. It has also been found that the majority of the contamination is within the first 3-10 mm of the length of the capillary tube, i.e., at its entrance end. Cleaning or replacement requires access to the capillary tube, which often requires breaking the vacuum maintained by the MS system. Hence, cleaning or replacing a contaminated capillary tube can require significant down-time in the operation of the MS system.
Therefore, there is a need for capillary-based ion transfer devices that more effectively address the problem of contamination. There is also a need for capillary-based ion transfer devices that provide improved evaporation of droplets and desolvation of the ions from the droplets. There is also a need for capillary-based ion transfer devices that allow careful control over the supersonic expansion occurring at the vacuum interface to reduce the associated cooling of the gas jet and potential for ion clustering. Reducing the exit velocity into the vacuum may also assist in creating a more stable gas flow in the vacuum chamber and more stable signal levels. There is also a need for capillary-based ion transfer devices having capillary entrance geometries at the higher-pressure side of the interface that are modified so as to change the gas flow near the capillary entrance and/or change the electric field shape at that location in a manner that improves ion capture and transmission and reduce contamination.